Stainless steel for use under circumstance where organic acid and saline are present

ABSTRACT

The present invention provides a stainless steel which is suitable for use in the food manufacturing plant, particularly a soy sauce manufacturing plant. A stainless steel to be used in the environment which contains organic acid and common salt comprising, C; 0.05 wt % or less, Si; 1.00 wt % or less, Mn; 1.00 wt % or less, P; 0.040 wt % or less, S; 0.03 wt % or less, Ni; 40.0 wt % or less, 16.0 wt %≦Cr≦26.0 wt %, 2.0 wt %≦Mo≦8.0 wt %, 0.05 wt %≦Al≦0.100 wt %, 0.10 wt %≦N≦0.30 wt %, Mg: 0.005 wt % or less, Ca; 0.0010 wt % or less and balance consisting of Fe and inevitable impurities, and satisfying equation (1), 
 
Cr+3.3Mo+20N≧38   (1) 
wherein, Cr, Mo and N show the content of each ingredients by weight %.

FIELD OF THE INVENTION

The present invention relates to a stainless steel which is superior in resistance to crevice corrosion and in resistance to stress corrosion cracking, and suited to the food manufacturing plant, preferably to the food manufacturing plant in the process of which organic acid such as amino acid, citric acid or acetic acid are generated and also the contents of common salt is high, in particular to the soy sauce manufacturing plant.

DESCRIPTION OF THE PRIOR ART

In general, for the food manufacturing plant, stainless steel, steel coated by inorganic or organic layer or FRP are used according to the contained ingredients in the foods to be handled, temperature or the operating conditions, especially, the use of stainless steel is increasing from the view point of the easy maintenance, low maintenance cost and excellent cleaning ability. In general, in the food manufacturing plant, such as soft drink, beer or milk, standard stainless steel, for example, SUS304 or SUS316 is mainly used, and the serious problem of leaking by corrosion does not occur. Further, even if in the case of foods containing common salt, if steels are used at around room temperature, the problem of localized corrosion such as pitting or crevice corrosion does not occur and can be used without anxiety. However, for example, in the case of manufacturing of a seasoning containing large amount of common salt e.g. soy sauce, serious localized corrosion problem occurs on SUS304 or SUS316 stainless steel even at room temperature, and the corrosion resistance is not enough. Further, in the case of use of SUS329 stainless steel which has better corrosion resistance than the above mentioned stainless steels, the localized corrosion rarely occurs, but when the temperature elevates higher than room temperature, since there is a possibility to generate the crevice corrosion or the stress corrosion cracking, the use of said stainless steel is limited. Therefore, in a soy sauce manufacturing plant, which is exposed to such a specific condition, inorganic or organic coated steel, FRP, further, nickel or titanium alloy which are more expensive than stainless steel are inevitably used.

SUMMARY OF THE INVENTION

The present invention is carried out according to the above mentioned circumstance, and the object of the present invention is to provide a stainless steel which is suitable for the food manufacturing plant, in particular, for a soy sauce manufacturing plant or a vinegar manufacturing plant in the fermentation process in which organic acid is generated and concentration of common salt is high.

The inventors of the present invention have conducted intensive study to investigate the stainless which is suited for the food manufacturing plant, in particular for a seasoning manufacturing plant for such as soy sauce having a fermentation process which contains large amount of common salt. And they found that in the case of fermentation process which generate organic acid such as amino acid, citric acid or lactic acid, these organic acid accelerated the corrosion of the stainless steel, especially the crevice corrosion or the stress corrosion cracking.

As the mechanism in which the corrosion of stainless steel is accelerated by organic acid, the inventors of the present invention have obtained the following knowledge. That is, amino acid, which is generated at the fermentation process, acts as the reducing agent and make deteriorate the surface passive film which provides corrosion resistance to stainless steel. While, citric acid or lactic acid acts the surface of stainless steel as a chelate and accelerate the dissolution of the water soluble oxide inclusions in steel such as CaO or MgO which are not covered by the surface passive film and the dissolved point becomes the origin of the crevice corrosion or the stress corrosion cracking. And the inventors of the present invention found, that as the first step, it was necessary to satisfy the following equation (1) for the purpose to improve the corrosion resistance of the surface passive film and the matrix metal. Cr+3.3Mo+20N≧38   (1) (in the formula, Cr, Mo and N show the content of (wt %) each ingredient.) Further, the inventors of the present invention have found that when the amount of CaO and MgO contained in the inclusion of the stainless steel is reduced and the main components of the inclusion in stainless steel is altered to SiO₂ or Al₂O₃, the corrosion resistance in the condition containing organic acid and large amount of common salt is improved. Namely, according to the experimental results, the inventors of the present invention have found that the generation of the crevice corrosion or the stress corrosion cracking, which is the main corrosion of the stainless steel, can be suppressed by satisfying following equation (2), Si+Al−100(Ca+Mg)≧0   (2) and by being the weight ratio of CaO+MgO in the inclusion is 20% or less, and accomplished the present invention.

The important point of the first invention of the present invention is the stainless steel to be used in the environment which contains organic acid and common salt comprising, C; 0.05wt % or less, Si; 1.00wt % or less, Mn; 1.00wt % or less, P; 0.040wt % or less, S; 0.03wt % or less, Ni; 40.0 wt % or less, 16.0 wt % ≦Cr≦26.0 wt %, 2.0 wt %≦Mo≦8.0 wt %, 0.05 wt %≦Al≦0.100 wt %, 0.10 wt %≦N≦0.30 wt %, Mg; 0.005 wt % or less, Ca; 0.0010 wt % or less and balance consisting of Fe and inevitable impurities, and satisfying following equation (1), Cr+3.3Mo+20N≧38   (1) (in the formula, Cr, Mo and N show the content (wt %) of each ingredients.) The important point of the second invention of the present invention is the austenitic stainless to be used in the environment which contains organic acid and common salt comprising, C; 0.05 wt % or less, Si; 1.00 wt % or less, Mn; 1.00 wt % or less, P; 0.040 wt % or less, S; 0.03 wt % or less, 15.0 wt %≦Ni≦40.0 wt %, 16.0 wt %≦Cr≦26.0 wt %, 2.0 wt %≦Mo≦8.0 wt %, 0.05 wt %≦Al≦0.100 wt %, 0.10 wt %≦N≦0.30 wt % and balance consisting of Fe and inevitable impurities, and satisfying following equation (1), Cr+3.3Mo+20N≧38   (1) (in the formula, Cr, Mo and N shows the content (wt %) in each ingredients.) The important of the third invention of the present invention is the stainless steel according to the first or the second invention of the present invention wherein the organic acid is at least one selected from the group consisting of amino acid, citric acid and lactic acid.

The important point of the fourth invention of the present invention is the stainless steel to be used in a food manufacturing plant comprising, C; 0.05 wt % or less, Si; 1.00 wt % or less, Mn; 1.00 wt % or less, P; 0.040 wt % or less, S; 0.03 wt % or less, Ni; 40.0 wt % or less, 16.0 wt %≦Cr≦26.0 wt %, 2.0 wt %≦Mo≦8.0 wt %, 0.05 wt %≦Al≦0.100 wt %, 0.10 wt %≦N≦0.30 wt %, Mg; 0.005 wt % or less, Ca; 0.0010 wt % or less and balance consisting of Fe and inevitable impurities, and satisfying following equation (1), Cr+3.3Mo+20N≧38   (1) (in the formula, Cr, Mo and N shows the content (wt %) in each ingredients.) The important point of the fifth invention of the present invention is the austenitic stainless steel to be used in a food manufacturing plant comprising, C; 0.05 wt % or less, Si; 1.00 wt % or less, Mn; 1.00 wt % or less, P; 0.040 wt % or less, S; 0.03 wt % or less, 15.0 wt %≦Ni≦40.0 wt %≦16.0 wt %≦Cr≦26.0 wt %, 2.0 wt %≦Mo≦8.0 wt %, 0.05 wt %≦A≦0.100 wt %, 0.10 wt %≦N≦0.30 wt % and balance consisting of Fe and inevitable impurities, and satisfying following equation (1), Cr+3.3Mo+20N≧38   (1) (in the formula, Cr, Mo and N shows the content (wt %) in each ingredients.) The important point of the sixth invention of the present invention is the austenitic stainless steel according to anyone of the inventions from the first to the fifth invention of the present invention, wherein the stainless steel satisfies the equation (2). Si+Al−100(Ca+Mg)≧0   (2) (in the equation, Si, Al, Ca and Mg shows the content (wt %) in each ingredients.) and by being the weight ratio of CaO+MgO in the inclusion to be 20% or less.

The important point of the seventh invention of the present invention is the stainless steel according to anyone of the inventions from the first to the sixth invention of the present invention, wherein the stainless steel is used for the soy sauce or vinegar manufacturing plant.

The important point of the eighth invention of the present invention is the stainless steel according to anyone of the inventions from the first to the seventh invention of the present invention, wherein the stainless steel further contains at least one selected from the group consisting of 0.01 wt %≦Cu≦1.0 wt %, 0.01 wt %≦W≦1.0 wt % and 0.01 wt %≦Co≦1.0 wt %.

The important point of the ninth invention of the present invention is the stainless steel according to anyone of the inventions from the first to the eighth invention of the present invention, wherein the stainless steel further contains 0.001 wt %≦B≦0.010 wt %.

In the stainless steel of the first and second inventions, it is desirable that the stainless steel further contains at least one selected from the group consisting of 0.01 wt %≦Cu≦1.0 wt %, 0.01 wt %≦W≦1.0 wt % and 0.01 wt %≦Co≦1.0 wt %, and it is more desirable that the stainless steel furthermore contains 0.001 wt %≦B≦0.010 wt %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the graph showing the AES analytical result of the surface of the test piece which is immersed in test solution for 1 week.

FIG. 2 is the graph showing the change of corrosion potential by time lapse when the test piece is dipped into test solution.

FIG. 3 is the graph showing the result of the corrosion test on test piece relating to the equation of Cr+3.3Mo+20N and CaO+MgO ratio in the inclusion.

FIG. 4 is the graph showing the relationship between the equation of Si+Al−100(Ca+Mg) and CaO+MgO ratio in the inclusion.

DESCRIPTION OF THE PREFERRED EMBOBYMENT

Th stainless steel of the present invention is constituted by (i) prescribed chemical composition and the limit of proper contents thereof, (ii) relationship between Cr, Mo and N which particularly contribute to the improvement of the corrosion resistance and (iii) component of inclusion in steel and limit of proper contents of Al, Si and Mg which consist said inclusion. The experimental results which are the base of the present invention are descrived as follows.

Experiment 1

As the first step, the inventors of the present invention investigated the difference between the environment in the manufacturing process of soy sauce which has a fermentation process and organic acid such as amino acid or lactic acid generate during said fermentation process and the environment in which such organic acids are not existing. In the experiments, commercial SUS316L of 2 mm in thickness was used as the specimen and two pieces which were cut to 20 mm×25 mm×2 mm and 60 mm×20 mm×2 mm were piled up and spot resistance welding was done at 4 points, thus the test piece with crevice for corrosion test was prepared. Although the ordinary soy sauce contains various kinds of organic acid, for the purpose to simplify the system, 4 kinds of test solution mentioned below were prepared. Namely, glutamic acid and asparagic acid, which are a kind of amino acid and are the typical organic acid generates at the fermentation process and lactic acid, citric acid and acetic acid which are not amino acid were added to the test solutions.

-   -   1-{circle over (1)}: 17% sodium chloride     -   1-{circle over (2)}: 17% sodium chloride+1% glutamic acid     -   1-{circle over (3)}: 17% sodium chloride+1% glutamic acid+1%         lactic acid     -   1-{circle over (4)}: 17% sodium chloride+1% glutamic acid+1%         asparagic acid+1% lactic acid+1% citric acid+0.15% acetic acid

These four test solutions were maintained at 35° C., and above mentioned test piece was respectively dipped into each test solutions and preserved for one month. After one month, the test pieces were cut by a cutter so as the cut line to pass the center of welding nugget part, and the cut surface was observed by optical microscope and the depth of crevice corrosion and the length of stress corrosion cracking were inspected. Results are summarized in Table 1. Corrosion generated by the test solution which contains sodium chloride alone (1-{circle over (1)})) is only crevice corrosion, while by the test solution which contains glutamic acid, a kind of amino acid, (1-{circle over (2)}) the generation of stress corrosion cracking is observed besides crevice corrosion. In the meanwhile, by the test solution containing lactic acid, which is not amino acid but possessing chelate structure together with glutamic acid (1-{circle over (3)}), the enlargement of the depth of crevice corrosion and length of stress corrosion cracking are observed. Further, by the test solution containing various organic acid (1-{circle over (4)}) the remarkable enlargement of corrosion is confirmed. According to the above mentioned results, it becomes clear that the environment in the manufacturing process of soy sauce in which organic acid such as amino acid or lactic acid generates at the fermentation process accelerate the corrosion remarkably compared with the environment in which organic acid is not exist, even if large amount of common salt is contained. TABLE 1 maximum maximum depth length of stress test of crevice corrosion solution ingredient corrosion cracking 1-{circle over (1)} 17% sodium chloride 14.5 μm — 1-{circle over (2)} 17% sodium chloride + 1% 15.5 μm 26.5 μm glutamic acid 1-{circle over (3)} 17% sodium chloride + 1% 39.0 μm 45.0 μm glutamic acid + 1% lactic acid 1-{circle over (4)} 17% sodium chloride + 1% 70.0 μm 71.0 μm glutamic acid + 1% asparagic acid + 1% lactic acid + 0.2% citric acid + 0.15% acetic acid Experiment 2

For the purpose to investigate the mechanism of the enhancement of corrosion by organic acid, the inventors of the present invention carried out the surface analysis of the SUS316L test piece which was dipped into high concentrated sodium chloride solution containing organic acid for long time and electrochemical measurement in said solution. Specifically, following 3 kinds of test solution were prepared and maintained at 35° C.

-   -   2-{circle over (1)}: 17% sodium chloride     -   2-{circle over (2)}: 17% sodium chloride+1% glutamic acid     -   2-{circle over (3)}: 17% sodium chloride+1% lactic acid

SUS316L flat plate test pieces polished with #400 emery grinding paper under wet condition were dipped into the above mentioned test solutions for 1 week, and the structural feature of the surface passive film was analyzed by Auger electron spectroscopic analyzer (hereinafter shortened to AES). And the surfaces of same test pieces were observed by a scanning electron microscope (hereinafter shortened to SEM). Further, the corrosion potential of each test pieces during one week dipping period was measured using a saturated calomel electrode as a reference electrode. Before the measurement of this corrosion potential, fresh air was previously blown into each test solutions so as the dissolved oxygen to be saturated.

The results of AES analysis on the surface of test pieces which are dipped into each test solutions for one week are summarized in FIG. 1. In FIG. 1, the numerical value which was arranged by fixing Ar accelerating potential to 1 kV and analyzing the constituting elements of surface passive film as [Cr]/[Cr]+[Fe] is shown, wherein, [Cr] and [Fe] shows respective atom%. Higher number of this indication, indicates stronger surface passive film, namely, indicates better corrosion resistance. As clearly understood from FIG. 1, in the cases of 17% sodium chloride (2-{circle over (1)}) or the solution (2-{circle over (3)}) prepared by adding 1% lactic acid to 17% sodium chloride, there is no difference in surface passive film structure. However, in the case of solution (2-{circle over (2)}) prepared by adding 1% glutamic acid to 17% sodium chloride, the value of [Cr]/[Cr]+[Fe] at the outermost surface is deteriorated when compared with 2-{circle over (1)} or 2-{circle over (1)}. This phenomenon indicates that glutamic acid acts to deteriorate the surface passive film. The measuring results of corrosion potential in each solution are shown in FIG. 2. In the cases of solution of 2-{circle over (1)} or 2-{circle over (3)}, the change of corrosion potential from the starting point of measurement is small, however, in the case of solution 2-{circle over (2)}, which contains glutamic acid, sudden deterioration of corrosion potential is observed immediately after the starting point of measurement. From the above results, it is understood that glutamic acid, which is amino acid, acts as the reducing agent, consequently makes the surface passive film instable.

In the meanwhile, these test pieces were dipped into each solutions for one week, then the surface of each test pieces were observed by SEM. The test pieces dipped into 2-{circle over (1)} and 2-{circle over (2)} did not change from the original, however, it is recognized that fine pores were formed on the surface of test piece dipped into 2-{circle over (3)} which contained lactic acid. These pores were formed at the position where inclusions exist. According to the careful observation, it became clear that, although the inclusions mainly composing of Al₂O₃ or SiO₂ existed also after dipping, the inclusions whose containing ratio of CaO or MgO was high were selectively dissolved. As the mechanism of this phenomenon, followings are considered. That is, since lactic acid has chelate structure, it reacts prior with Ca or Mg, which has strong affinity with lactic acid, consequently, selectively dissolves the inclusions of CaO or MgO and becomes the starting point of crevice corrosion or stress corrosion cracking. Therefore, by the above mentioned reason, it is understood that among the organic acids, lactic acid or citric acid which has chelate structure enhance the corrosion, further, when the inclusions mainly composing of CaO or MgO exist, corrosion resistance is deteriorated.

Experiment 3

The peculiarity of the environment of soy sauce manufacturing plant containing common salt in high concentration in which organic acid exists, the deterioration of the surface passive film by organic acid and the mechanism to enhance the corrosion by selective dissolution of the inclusions mainly composed of CaO or MgO are recited above. As the next step, the inventors of the present invention carried out the following experiment aiming to find out the proper composition of the stainless steel which exhibits good corrosion resistance in such a particular environment. Stainless steel, whose chemical composition is in the range mentioned below, was produced by an induction furnace so as the weight ratio of CaO+MgO in oxide inclusions in steel to be variously changed, and ingots were obtained.

C: 0.008-0.035 wt %, Si: 0.02-0.24 wt %, Mn: 0.13-0.92 wt %, P: 0.017-0.034 wt %, S: 0.001-0.003 wt %, Ni: 6.44-34.83 wt %, Cr: 16.51-25.12 wt %, Mo: 2.06-7.47 wt %, Cu: 0.01-0.86 wt %, W: 0.01-0.73 wt %, Co: 0.01-0.75 wt %, Al: 0.006-0.92 wt %, N: 0.02-0.30 wt %, Ca: 0.0001-0.0052 wt %, Mg: 0.0001-0.0018 wt % and B: 0.0001-0.0036 wt %.

To ingots, were heat treated at 1250° C. for 8 hours, forged, cold rolled to 2 mm in thickness and then solution heat treated at 1150° C. for 30 minutes followed by water cooling. Then test pieces were cut from the cold rolled plates of 2 mm in thickness by the same way as Experiment 1 and prepared test pieces with crevice by spot welding. Soy sauce, which was the fermentation seasoning containing 17% sodium chloride, was used as the test solution, and above mentioned test pieces were dipped into said test solution for 5 months maintaining the temperature at 35° C. After dipping, the test pieces were cut by a cutter so as the cut line to pass the center of welding nugget part, and the cut surface was observed by optical microscope and the generation of crevice corrosion or stress corrosion cracking were inspected. When any corrosion is observed, evaluation is ×, and when no corrosion is observed evaluation is ◯.

The corrosion test results of the materials whose weight ratio of CaO+MgO in oxide inclusions in steel is 20% or less and the materials whose weight ratio of CaO+MgO is bigger than 20% are respectively shown in FIG. 3. In the horizontal axis of FIG. 3, Cr, Mo and N, which have large influence to corrosion, are selected and the value Cr+3.3Mo+20N (wherein Cr, Mo and N are the content wt % of each elements) which is dignified so as the influence of each elements to become nearly equal is indicated. From FIG. 3, it is recognized that in the case of weight ratio of CaO+MgO in oxide inclusions in steel is bigger than 20%, corrosion does not occur when the value of Cr+3.3Mo+20N exceed 44. On the contrary, in the case of weight ratio of CaO+MgO in oxide inclusions in steel is 20% or less, corrosion does not occur when the value of Cr+3.3Mo+20N exceed 38. It is obvious that when the value of Cr+3.3Mo+20N is bigger, the corrosion resistance becomes better, however, it becomes necessary to add expensive elements in alloy and causes the cost elevation. By controlling the weight ratio of CaO+MgO in oxide inclusions in steel to 20% or less, the lower limit of Cr+3.3Mo+20N necessary to the corrosion resistance can be dropped. However, even if said control is carried out, it is necessary that the indication of Cr+3.3Mo+20N to be at least 38. When this indication is smaller than 38, there is a possibility to cause corrosion in materials used in a say sauce manufacturing plant containing high concentration of sodium chloride and organic acid.

Then the inventors of the present invention, carried out the investigation to control the weight ratio of CaO+MgO in the oxide inclusions in steel to 20% or less in stable, and found out that the said value of ratio could be accomplished by considering migration of Ca and Mg from furnace bricks, and adjust the contents of Si and Al, which are the components of a deoxidation, in a specified range. That is, as shown in FIG. 4, by setting up the contents of Si and Al respectively to 0.01-0.25 wt % and 0.005-0.100 wt % and by making the content of Ca and Mg satisfied with the equation of Si+Al−100(Ca+Mg)≧0, weight ratio of CaO+MgO in the inclusions can be maintained in stable to 20% or less. As mentioned above, it is found out that, by controlling component range of Cr, Mo, N, Si and Al and constitution of the inclusions, the austenitic stainless steel having good corrosion resistance in a say sauce manufacturing plant containing common salt and organic acid in high concentration.

The grounds for limitation of each components are explained as follows.

C: 0.05 wt % or less

Since C is characterized as an element which cause sensitization by the welding and deteriorate the corrosion resistance, it is desirable that the content to be low, however, the remarkable decrease of C brings about the deterioration of strength, further brings about the cost elevation. Since the maximum permissible content of C is 0.05 wt %, this value is set up as the upper limit.

Si: 1.00 wt % or less

Si is an element which is useful for desoxidation, in particular, is a desirable element to decrease the CaO+MgO ratio in the oxide inclusions in steel and composes main component of the oxide inclusions with Al, however, the excess addition causes the saturation of the effect, further causes deterioration of ductility and increase of strength, and encourage the precipitation of intermetallic compound such as σ phase or χ phase and deteriorate the corrosion resistance. Therefore, it is necessary to control the content of Si to 1.0 wt % or less. Desirably, less than 0.70%, 0.5%, 0.25%, 0.20%, more desirably less than 0.10 wt %.

Mn: 1.00 wt % or less

Mn is an element necessary to be decreased to defer the precipitation of intermetallic compound such as σ phase or χ phase and to suppress the deterioration of the corrosion resistance, and is necessary to adjust the content to 1.00 wt % or less. Desirable limit is 0.30 wt % or less, more desirable limit is 0.20 wt % or less.

P: 0.04 wt % or less

P is an element which is inevitably contained as an impurity and segregates in grain boundary, and lower content is desirable from the view points of corrosion resistance and hot workability. However, the remarkable decrease in P content brings about the cost elevation. Since the maximum permissible content of P is 0.040 wt %, this value is set up as the upper limit. However, the desirable upper limit of P is 0.03 wt % or less.

S: 0.003 wt % or less

Same as to P, S is an element which is inevitably contained as an impurity and segregates in grain boundary, and lower content is desirable from the view points of corrosion resistance and hot workability. In particular, when the content exceeds 0.003 wt %, since the harmfulness of it is displayed remarkably, the limit of content is set up to 0.003 wt % or less. However, the desirable upper limit of S is 0.002 wt % or less.

Ni: 40.0 wt % or less

Ni is an element which is effective to control the precipitation of intermetallic compound such as o r phase or X phase and also is a necessary element to form the austenitic structure. Further, Ni is the effective element to improve the resistance against stress corrosion cracking, however, when the content of Ni exceed 40.0 wt %, brings about the deterioration of hot workability or the increase in high temperature strength. Therefore, the upper limit of Ni content is set up to 40.0 wt %. The desirable content of Ni is from 18.0 to 30 wt %, and more desirably is from 24.0 to 26 wt %.

Cr: 16.0 wt %≦Cr≦26.0 wt %

Cr is an effective element to improve the crevice corrosion resistance, and is necessary to be contained by 16.0 wt % or more to display said effect. However, when the content exceeds 26.0 wt %, since it encourages the precipitation of intermetallic compound such as σ phase or χ phase and deteriorate the crevice corrosion resistance, the limit is set up from 16.0 wt % to 26.0 wt %. Further, the desirable limit of Cr content is 20.0 wt % or more, and more desirable limit of Cr content is 22.0 wt % or more.

Mo: 2.0 wt %≦Mo≦8.0 wt %

Mo is also an effective element to improve the crevice corrosion resistance, and is necessary to be contained by 2.0 wt % or more to display said effect. However, when the content exceeds 8.0 wt %, since it encourages the precipitation of intermetallic compound and deteriorate the corrosion resistance, the limit is set up from 2.0 wt % to 8.0 wt %. Further, the desirable limit of Mo content is 3.0 wt % or more, and more desirable limit of Mo content is 5.0 wt % or more.

Al: 0.005 wt %≦Al≦0.100 wt %

Al is a strong deoxydation element, and as shown in Experiment 3, it is necessary to add possitively for the purpose to decrease the CaO+MgO ratio in the oxide inclusions in steel and composes main component of the oxide inclusions together with Si, however, when the content exceeds 0.10 wt %, since the effect is saturated and encourages the precipitation of intermetallic compound, the limit of content of Al is set up to 0.10 wt % or less.

N: 0.10 wt %≦N≦0.30 wt %

Same as to Cr or Mo, N is an element to improve the crevice corrosion resistance, further is the effective element to suppress the precipitation of intermetallic compound, and it is necessary to contain 0.10 wt % or more to display said effect. However, when the content exceeds 0.30 wt %, since high temperature strength increases remarkably and deteriorates the hot workability, the limit of N content is set up from 0.10 wt % to 0.30 wt %. Desirably the limit of content of N is 0.15 wt % or more.

Mg: 0.005 wt % or less

Although Mg is an element which is inevitably contained in ordinary oxide inclusions in steel, obviously from the result of Experiment 3, it is necessary to limit the content of it to 0.005 wt % or less from the view point of the corrosion resistance. That is, when the content of Mg exceeds 0.005 wt %, it becomes possible to form an inclusion which is soluble in organic acid having chelate structure and causes deterioration of corrosion resistance.

Ca: 0.0010 wt % or less

Although Ca is an element which is inevitably contained in ordinary oxide inclusions in steel like as to Mg, obviously from the result of Experiment 3, it is necessary to limit the content of it to 0.0010 wt % or less from the view point of the corrosion resistance. That is, when the content of Ca exceeds 0.0010 wt %, it becomes possible to form an inclusion which is soluble in organic acid having chelate structure and causes deterioration of corrosion resistance.

Cu: 0.01-1.0 wt %

W: 0.01-1.0 wt %

Co: 0.01-1.0 wt %

In the present invention, it is possible to contain at least one selected from the group consisting of 0.01 wt %≦Cu−1.0 wt %, 0.01 wt %≦W≦1.0 wt % and 0.01 wt %≦Co≦1.0 wt % besides above mentioned components. These elements are effective to improve general corrosion resistance, and it is necessary to be contained by 0.01 wt % or more to exhibit the effect. On the contrary, when the content exceeds 1.0 wt %, since the hot workability is deteriorated, the limits of each elements are set up to 0.01-1.0 wt %.

B: 0.01 wt %≦B≦0.010 wt %

In the present invention, it is possible to contain 0.001 wt %≦B≦0.010 wt % besides above mentioned components. B is very useful for the improvement of hot workability, however, when the content of B is less than 0.001 wt %, the effect is not sufficient enough, and when the content of B exceeds 0.010 wt %, the hot workability is deteriorated. Therefore the limit of content of B is set up to 0.001 wt % to 0.10 wt %. Cr+3.3Mo+20N≧38

In the present invention, the reason why Cr, Mo and N are limited to the following equation, Cr+3.3Mo+20N≧38 (wherein Cr, Mo and N are content (wt %) of each elements) is that, as obviously understood from the result of Experiment 3, when the value of Cr+3.3Mo+20N is smaller than 38, even if the weight ratio of CaO+MgO in the oxide inclusions in steel, which is the main constituting factor of the present invention, is controlled by optimization of contents of Si, Al, Ca and Mg, it does not have a sufficient corrosion resistance in a soy sauce manufacturing plant containing high concentration of common salt and organic acid. The desirable limit of value of Cr+3.3Mo+20N is 40 or more, and more desirably is 44 or more.

The weight ratio of CaO+MgO in oxide inclusions in steel is 20% or less Si+Al−100(Ca+Mg)≧O

In the present invention, the reason why the weight ratio of CaO+MgO in oxide inclusions in steel is limited to 20% or less, and Si, Al, Ca and Mg are limited by following equation, Si+Al−100(Ca+Mg)≧0 (wherein Si, Al, Ca and Mg are content (wt %) of each elements) is that, as obviously understood from the results of Experiment 3, when CaO+MgO in the oxide inclusions in steel is over than 20% and above mentioned equation is not satisfied, the good corrosion resistance can not be obtained in a soy sauce manufacturing plant containing high concentration of sodium chloride and organic acid. In the present invention, it is not necessary that the all of oxide inclusions in steel to be SiO₂, Al₂O₃, CaO or MgO alone or complex oxide thereof, but is sufficient for any inclusion only to satisfy the ratio of CaO+MgO being 20% or less. As a matter of cause, there is a case that other oxides form complex oxide by alone or with the above mentioned oxides. As the other oxides, for example, MnO, FeO and TiO₂ can be mentioned.

EXAMPLES AND COMPARATIVE EXAMPLES

The present invention will be illustrated more in detail according to the following Examples. While, all kinds of steel shown in Experiment 3 are mentioned as well.

As the first step, the steels of the present invention and the steels for comparison having chemical composition shown in Tables 2 and 3 were produced in an induction furnace and ingots were obtained. The ingots were heat treated at 1250° C. for 8 hours, forged, cold rolled to 2 mm in thickness and then, solution heat treated at 1150° C. for 30 minutes followed by water cooling.

Then, two test pieces were cut from the cold rolled plate of 2 mm in thickness, polished with #400 emery paper under wet condition and degreased. After that, test pieces with crevice were prepared by spot welding.

Soy sauce, which is the fermentation seasoning containing 17% sodium chloride, is used as the test solution, and above mentioned test pieces were immersed in said test solution for 5 months maintaining the temperature at 35° C. After immersing, the test pieces were cut by a cutter so as the cut line to pass the center of welding nugget part, and the cut surface is observed by optical microscope and the generation of crevice corrosion or stress corrosion cracking were inspected. Results are summarized in Table 3.

◯ mark: Crevice corrosion or stress corrosion cracking are not generated and material shows good corrosion resistance

× Mark: Both crevice corrosion and stress corrosion cracking are generated Results are summarized in Table 3. TABLE 2 No C Si Mn P S Ni Cr Mo Al N Ca Mg Cu W Co B Steel 1 0.035 0.12 0.23 0.021 0.001 15.17 16.51 4.77 0.012 0.29 0.0003 0.0002 — — — — of the 2 0.011 0.11 0.13 0.022 0.001 18.45 19.37 5.83 0.038 0.12 0.0007 0.0001 — — — — present 3 0.008 0.02 0.18 0.019 0.001 18.84 20.26 6.09 0.092 0.21 0.0006 0.0002 — — — 0.0033 invention 4 0.009 0.23 0.20 0.020 0.002 19.31 20.41 6.24 0.041 0.23 0.0002 0.0002 0.71 — — — 5 0.011 0.20 0.18 0.020 0.001 24.56 20.97 6.05 0.019 0.22 0.0002 0.0001 — — — 0.0026 6 0.010 0.18 0.26 0.021 0.001 24.98 21.02 6.76 0.026 0.20 0.0003 0.0001 0.32 — — — 7 0.010 0.24 0.20 0.019 0.001 29.23 23.03 7.47 0.044 0.22 0.0009 0.0002 — — — 0.0036 8 0.006 0.09 0.21 0.018 0.001 25.85 23.13 5.34 0.052 0.19 0.0003 0.0004 — — — 0.0031 9 0.007 0.06 0.20 0.020 0.001 25.78 23.31 5.53 0.017 0.20 0.0004 0.0003 — — — 0.0022 10 0.009 0.13 0.20 0.017 0.002 24.57 20.14 6.18 0.019 0.22 0.0006 0.0002 0.86 — — 0.0041 11 0.009 0.21 0.21 0.019 0.001 25.03 22.87 5.72 0.023 0.19 0.0008 0.0005 — 0.73 — — 12 0.010 0.17 0.17 0.022 0.001 25.11 20.63 6.02 0.034 0.22 0.0001 0.0003 — — 0.75 — 13 0.011 0.18 0.38 0.034 0.003 24.89 26.84 2.06 0.063 0.30 0.0003 0.0002 — — — — 14 0.009 0.23 0.23 0.019 0.001 6.55 24.60 3.22 0.035 0.17 0.0004 0.0002 — 0.16 — 0.0017 15 0.010 0.20 0.34 0.020 0.001 6.76 25.12 3.41 0.029 0.16 0.0005 0.0001 — 0.34 — — Com- 16 0.013 0.18 0.22 0.018 0.003 10.81 16.78 2.13 0.006 0.02 0.0008 0.0002 — — — — parative 17 0.010 0.17 0.19 0.019 0.002 15.04 18.13 3.98 0.008 0.15 0.0036* 0.0004 — — — — Ex. 18 0.011 0.24 0.16 0.019 0.001 13.67 19.24 3.56 0.016 0.06 0.0009 0.0002 — — — — 19 0.012 0.17 0.20 0.018 0.001 25.46 20.74 5.02 0.009 0.15 0.0036* 0.0004 — — — — 20 0.011 0.25 0.19 0.020 0.001 24.95 20.87 5.37 0.026 0.17 0.0052* 0.0018* — — — — 21 0.010 0.14 0.20 0.020 0.001 24.64 24.26 2.31 0.037 0.26 0.0006 0.0003 — — — — 22 0.008 0.21 0.18 0.017 0.001 6.44 24.72 3.16 0.037 0.17 0.0018* 0.0009* — — — — Numerical value in the Table is Wt %, balance is Fe, *not included in the present invention

TABLE 3 CaO + MgO corrosive test Si + Al − 100 ratio (%) in result in soy No. Cr + 3.3 Mo + 20 N (Ca + Mg) inclusion sauce steel of the 1 38.05 0.082 15.1 ◯ present 2 41.01 0.068 12.6 ◯ invention 3 44.56 0.032 13.5 ◯ 4 45.60 0.231 <0.1 ◯ 5 45.34 0.189 0.2 ◯ 6 47.33 0.166 1.6 ◯ 7 52.08 0.174 2.5 ◯ 8 44.55 0.072 18.3 ◯ 9 45.56 0.007 16.2 ◯ 10 44.93 0.069 9.4 ◯ 11 45.55 0.103 11.8 ◯ 12 44.90 0.164 4.0 ◯ 13 39.63 0.193 3.2 ◯ 14 38.63 0.205 <0.1 ◯ 15 39.57 0.169 6.8 ◯ steel for 16 24.21* 0.086 14.9 X comparison 17 34.26* −0.002* 25.3 X 18 32.19* 0.146 6.0 X 19 40.31 −0.221* 36.3 X 20 41.99 −0.424* 95.1 X 21 37.09* 0.087 10.2 X 22 38.55 −0.081* 23.7 X *not included in the present invention

In Table 3, indexes of Cr+3.3Mo+20N and Si+Al−100(Ca+Mg) and the average weight ratio (%) of CaO+MgO in an oxide inclusion in steel are shown as well. Steels of the present invention of Cr+3.3Mo+20N ≧38 and Si+Al−100(Ca+Mg)≧0, further the weight ratio of CaO+MgO in oxide inclusion is steel is 20% or less, do not generate corrosion in soy sauce environment which contains high concentration of sodium chloride and organic acid and display excellent corrosion resistance compared with steels for comparison.

The steels of the present invention and the steels for comparison having constitution shown in Tables 4 and 5, which change the amount of Mn were produced by same manner as those shown in Table 2. Then two test pieces of 80×25×2 mm and 60×20×2 mm were prepared by same manner as Table 2.

The corrosion test was carried out same as to above Examples and the results are shown in Table 5. Similar results are obtained. TABLE 4 No C Si Mn P S Ni Cr Mo Al N Ca Mg Cu W Co B Steel 1 0.03 0.12 0.50 0.021 0.001 15.17 16.51 4.77 0.012 0.29 0.0003 0.0002 — — — — of the 2 0.01 0.11 0.60 0.022 0.001 18.45 19.37 5.83 0.038 0.12 0.0007 0.0001 — — — — present 3 0.00 0.02 0.70 0.019 0.001 18.84 20.26 6.09 0.092 0.21 0.0006 0.0002 — — — 0.0033 invention 4 0.00 0.23 0.60 0.020 0.002 19.31 20.41 6.24 0.041 0.23 0.0002 0.0002 0.71 — — — 5 0.01 0.20 0.52 0.020 0.001 24.56 0.97 6.05 0.019 0.22 0.0002 0.0001 — — — 0.0026 6 0.01 0.18 0.53 0.021 0.001 24.98 21.02 6.76 0.026 0.20 0.0003 0.0001 0.32 — — 7 0.01 0.24 0.52 0.019 0.001 29.23 23.03 7.47 0.044 0.22 0.0009 0.0002 — — — 0.0036 8 0.00 0.09 0.57 0.018 0.001 25.85 23.13 5.34 0.052 0.19 0.0003 0.0004 — — — 0.0031 9 0.00 0.06 0.92 0.020 0.001 25.78 23.31 5.53 0.017 0.20 0.0004 0.0003 — — — 0.0022 10 0.00 0.13 0.60 0.017 0.002 24.57 20.14 6.18 0.019 0.22 0.0006 0.0002 0.86 — — 0.0041 11 0.00 0.21 0.53 0.019 0.001 25.03 22.87 5.72 0.023 0.19 0.0008 0.0005 — 0.73 — — 12 0.01 0.17 0.51 0.022 0.001 25.11 20.63 6.02 0.034 0.22 0.0001 0.0003 — — 0.75 — 13 0.01 0.18 0.55 0.034 0.003 24.89 26.84 2.06 0.063 0.30 0.0003 0.0002 — — — — 14 0.00 0.50 0.52 0.019 0.001 6.55 24.60 3.22 0.035 0.17 0.0004 0.0002 — 0.16 — 0.0017 15 0.01 0.70 0.54 0.020 0.001 6.76 25.12 3.41 0.029 0.16 0.0005 0.0001 — 0.34 — — Com- 16 0.01 0.18 0.54 0.018 0.003 10.81 16.78 2.13 0.006 0.02 0.0008 0.0002 — — — — parative 17 0.01 0.17 0.54 0.019 0.002 15.04 18.13 3.98 0.008 0.15 0.0036* 0.0004 — — — — Ex. 18 0.01 0.24 0.55 0.019 0.001 13.67 19.24 3.56 0.016 0.06 0.0009 0.0002 — — — — 19 0.01 0.17 0.57 0.018 0.001 25.46 20.74 5.02 0.009 0.15 0.0036* 0.0004 — — — — 20 0.01 0.25 0.56 0.020 0.001 24.95 20.87 5.37 0.026 0.17 0.0052* 0.0018* — — — — 21 0.01 0.14 0.52 0.020 0.001 24.64 24.26 2.31 0.037 0.26 0.0006 0.0003 — — — — 22 0.00 0.21 0.45 0.017 0.001 6.44 24.72 3.16 0.037 0.17 0.0018* 0.0009* — — — — Numerical value in the Table is Wt %, balance is Fe. *not included in the present invention

TABLE 5 CaO + MgO corrosive test Si + Al − 100 ratio (%) in result in soy No. Cr + 3.3 Mo + 20 N (Ca + Mg) inclusion sauce steel of the 1 38.05 0.082 15.1 ◯ present 2 41.01 0.068 12.6 ◯ invention 3 44.56 0.032 13.5 ◯ 4 45.60 0.231 <0.1 ◯ 5 45.34 0.189 0.2 ◯ 6 47.33 0.166 1.6 ◯ 7 52.08 0.174 2.5 ◯ 8 44.55 0.072 18.3 ◯ 9 45.56 0.007 16.2 ◯ 10 44.93 0.069 9.4 ◯ 11 45.55 0.103 11.8 ◯ 12 44.90 0.164 4.0 ◯ 13 39.63 0.193 3.2 ◯ 14 38.63 0.475 <0.1 ◯ 15 39.57 0.669 6.8 ◯ steel for 16 24.21* 0.086 14.9 X comparison 17 34.26* −0.002* 25.3 X 18 32.19* 0.146 6.0 X 19 40.31 −0.221* 36.3 X 20 41.99 −0.424* 95.1 X 21 37.09* 0.087 10.2 X 22 38.55 −0.081* 23.7 X *not included in the present invention Industrial Applicability

As illustrated above, in the stainless steel of the present invention, since the total weight of Cr, Mo and N is specifically dignified and lower limit is set up, further the limit of Si, Al, Ca and Mg are set up so as to control the constitution of oxide inclusions in steel, it becomes possible to develop the stainless steel which has excellent corrosion resistance to foods plant, especially soy sauce manufacturing plant containing high concentrated sodium chloride and organic acid generates at fermentation process. 

1-9. (Cancelled)
 10. A stainless steel to be used in the environment which contains organic acid and common salt comprising, C; 0.05 wt % or less, Si; 1.00 wt % or less, Mn; 1.00 wt % or less, P; 0.040 wt % or less, S; 0.03 wt % or less, Ni; 40.0 wt % or less, 16.0 wt %≦Cr≦26.0 wt %, 2.0 wt %≦Mo≦8.0 wt %, 0.05 wt %≦Al≦0.100 wt %, 0.10 wt %≦N≦0.30 wt %, Mg: 0.005 wt % or less, Ca; 0.0010 wt % or less and balance consisting of Fe and inevitable impurities, and satisfying equation (1), Cr+3.3Mo+20N≧38   (1) wherein, Cr, Mo and N show the content of each ingredients by weight %.
 11. The stainless steel of claim 10, wherein the organic acid is at least one selected from the group consisting of amino acid, citric acid and lactic acid.
 12. An austenitic stainless steel to be used in the environment which contains organic acid and common salt comprising, C; 0.05 wt % or less, Si; 1.00 wt % or less, Mn; 1.00 wt % or less, P; 0.040 wt % or less, S; 0.03 wt % or less, 15.0 wt %≦Ni≦40.0. wt %, 16.0 wt %≦Cr≦26.0 wt %, 2.0 wt %≦Mo≦8.0 wt %, 0.05 wt %≦Al≦0.100 wt %, 0.10 wt %≦N≦0.30 wt % and balance consisting of Fe and inevitable impurities, and satisfying equation (1), Cr+3.3Mo+20N≧38   (1) wherein, Cr, Mo and N show the content of each ingredients by weight %.
 13. The stainless steel of claim 12, wherein the organic acid is at least one selected from the group consisting of amino acid, citric acid and lactic acid.
 14. A stainless steel to be used in a food manufacturing plant comprising, C; 0.05 wt % or less, Si; 1.00 wt % or less, Mn; 1.00 wt % or less, P; 0.040 wt % or less, S; 0.03 wt % or less, Ni; 40.0 wt % or less, 16.0 wt %≦Cr≦26.0 wt %, 2.0 wt %≦Mo≦8.0 wt %, 0.05 wt %≦Al≦0.0010 wt %, 0.10 wt %≦N≦0.30 wt %, Mg: 0.005 wt % or less, Ca; 0.0010 wt % or less and balance consisting of Fe and inevitable impurities, and satisfying equation (1), Cr+3.3Mo+20N≧38   (1) wherein, Cr, Mo and N show the content of each ingredients by weight %.
 15. An austenitic stainless steel to be used in a food manufacturing plant comprising, C; 0.05 wt % or less, Si; 1.00 wt % or less, Mn; 1.00 wt % or less, P; 0.040 wt % or less, S; 0.03 wt % or less, 15.0 wt %≦Ni≦40.0. wt %, 16.0 wt % <Cr <26.0 wt %, 2.0 wt %≦Mo≦8.0 wt %, 0.05 wt %≦Al≦0.100 wt %, 0.10 wt %≦N≦0.30 wt % and balance consisting of Fe and inevitable impurities, and satisfying equation (1), Cr+3.3Mo+20N≧38   (1) wherein, Cr, Mo and N show the content of each ingredients by weight %.
 16. (New) The austenitic stainless steel according to claim 10, wherein the stainless steel satisfies the equation (2), Si+Al−100(Ca+Mg)≧0   (2) wherein Si, Al, Ca and Mg show the content of each ingredients by weight %.
 17. The stainless steel according to claim 10, wherein the stainless steel is used in the soy sauce or vinegar manufacturing plant.
 18. The stainless steel according to claim 10, wherein the stainless steel further contains at least one selected from the group consisting of 0.01 wt %≦Cu≦1.0 wt %, 0.01 wt %≦W≦1.0 wt % and 0.01 wt %≦Co≦1.0 wt %.
 19. The stainless steel according to claim 10, wherein the stainless steel further contains 0.001 wt %≦B≦0.010 wt %.
 20. The austenitic stainless steel according to claim 12, wherein the stainless steel satisfies the equation (2), Si+Al−100(Ca+Mg)≧0   (2) wherein Si, Al, Ca and Mg show the content of each ingredients by weight %.
 21. The stainless steel according to claim 12, wherein the stainless steel is used in the soy sauce or vinegar manufacturing plant.
 22. The stainless steel according to claim 12, wherein the stainless steel further contains at least one selected from the group consisting of 0.01 wt %≦Cu≦1.0 wt %, 0.01 wt %≦W≦1.0 wt % and 0.01 wt %≦Co≦1.0 wt %.
 23. The stainless steel according to claim 12, wherein the stainless steel further contains 0.001 wt %≦B≦0.010 wt %.
 24. The austenitic stainless steel according to claim 14, wherein the stainless steel satisfies the equation (2), Si+Al−100(Ca+Mg)≧0   (2) wherein Si, Al, Ca and Mg show the content of each ingredients by weight %.
 25. The stainless steel according to claim 14, wherein the stainless steel is used in the soy sauce or vinegar manufacturing plant.
 26. The stainless steel according to claim 14, wherein the stainless steel further contains at least one selected from the group consisting of 0.01 wt %≦Cu≦1.0 wt %, 0.01 wt %≦W≦1.0 wt % and 0.01 wt %≦Co≦1.0 wt %.
 27. The stainless steel according to claim 10, wherein the stainless steel further contains 0.001 wt %≦B≦0.010 wt %.
 28. The austenitic stainless steel according to claim 15, wherein the stainless steel satisfies the equation (2), Si+Al−100(Ca+Mg)≧0   (2) wherein Si, Al, Ca and Mg show the content of each ingredients by weight %.
 29. The stainless steel according to claim 15, wherein the stainless steel is used in the soy sauce or vinegar manufacturing plant.
 30. The stainless steel according to claim 15, wherein the stainless steel further contains at least one selected from the group consisting of 0.01 wt %≦Cu≦1.0 wt %, 0.01 wt %≦W≦1.0 wt % and 0.01 wt %≦Co≦1.0 wt %.
 31. The stainless steel according to claim 15, wherein the stainless steel further contains 0.001 wt %≦B≦0.010 wt %. 